ECE544: Communication Networks-II Spring 2010

1. ECE544: Communication Networks-II Spring 2010 D. Raychaudhuri Lecture 6 Includes teaching materials from L. Peterson, J. Kurose, K. Almeroth

2. IP Multicast Introduction Internet Group Management Protocol (IGMP) Routing Protocols Intra-domain (DVMRP, MOSPF, PIM) Inter-domain (MBGP, MSDP)

3. Multicast: act of sending datagram to multiple receivers with single “transmit” operation One-to-many, many-to-many Question: how to achieve multicast Multicast: one sender to many receivers • Multicast via unicast • source sends N unicast datagrams, one addressed to each of N receivers • Redundant traffic around sender • Keep track of all the IP addresses to send to Routers forward unicast datagrams multicast receiver (red) not a multicast receiver

4. Multicast: act of sending datagram to multiple receivers with single “transmit” operation One-to-many, many-to-many Question: how to achieve multicast Multicast: one sender to many receivers • Network multicast • (IP Multicast) • Routers actively participate in multicast, making copies of packets as needed and forwarding towards multicast receivers Multicast routers (red) duplicate and forward multicast datagrams

5. Multicast: act of sending datagram to multiple receivers with single “transmit” operation One-to-many, many-to-many Question: how to achieve multicast Multicast: one sender to many receivers • Application-layer multicast (P2P) • end systems (“hosts”) involved in multicast copy and forward unicast datagrams among themselves • “host” becomes router P2P hosts duplicate and forward multicast datagrams

6. Internet Multicast Service Model 128.59.16.12 multicast group concept: • Each group has its own IP multicast address • A host can join or leave freely • Routers forward multicast datagrams (with destination address of the group’s multicast address) to hosts that have “joined” that multicast group 128.119.40.186 multicast group 226.17.30.197 128.34.108.63 128.34.108.60

7. Multicast groups • class D Internet addresses reserved for multicast: • host group semantics: • anyone can “join” (receive) or leave multicast group • anyone (not even a member) can send to multicast group • no network-layer identification of hosts members • needed: infrastructure to deliver mcast-addressed datagrams to all hosts that have joined that multicast group

8. Mapping IP Multicast Address to Ethernet Address • Ethernet MAC Addresses: 48 bits • broadcast: all 1s, ff:ff:ff:ff:ff:ff • multicast: multicast flag (the lowest bit of the 1st octet)= 1 • 01-00-5E-00-00-00 to 01-00-5E-7F-FF-FF for IP multicast • IP multicast group address mapped to the lower order 23 bits of MAC address • not one-to-one mapping, one Ethernet mcast addr  32 IP mcast addrs

9. IPv6 Multicast Addresses (RFC 2375) • low-order flag indicates permanent / transient group; three other flags reserved • scope field: 1 - node local • 2 - link-local • 5 - site-local • 8 - organization-local • B - community-local • E - global • (all other values reserved) flags scope group ID 11111111 8 4 4 112 bits

10. Joining a mcast group: two-step process • local: host informs local mcast router of desire to join group: IGMP (Internet Group Management Protocol) • wide area: local router interacts with other routers to receive mcast datagram flow • many protocols (e.g., DVMRP, MOSPF, PIM) IGMP IGMP wide-area multicast routing IGMP

11. IGMP: Internet Group Management Protocol • host: sends IGMP report when application joins mcast group • IP_ADD_MEMBERSHIP socket option • host need not explicitly “unjoin” group when leaving • router:sends IGMP query at regular intervals • host belonging to a mcast group must reply to query report query

12. How IGMP Works • on each link, one router is elected the “querier” • querier periodically sends a Membership Query messageto the all-systems group (224.0.0.1), with TTL = 1 • on receipt, hosts start random timers (between 0 and 10 seconds) for each multicast group to which they belong routers: Q hosts:

13. How IGMP Works (cont.) • when a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1 • other members of G hear the report and stop their timers • routers hear all reports, and time out non-responding groups Q G G G G

14. Source Specific Multicast • Source Specific Multicast: a receiving host specifies (source, mcast group) to join • receive multicast packets addressed to the group and only if they are from the specific sender (one-to-many) • Any source multicast (ASM): many-to-many

15. IGMP version 1 router: Host Membership Query msg broadcast on LAN to all hosts host: Host Membership Report msg to indicate group membership randomized delay before responding implicit leave via no reply to Query RFC 1112 IGMP v2:additions include group-specific Query Leave Group msg last host replying to Query can send explicit Leave Group msg router performs group-specific query to see if any hosts left in group RFC 2236 IGMP v3: Join/Leave specific S in G RFC 3376 IGMP

16. Source-based trees Shared tree Multicast Routing: Problem Statement • Goal: find a tree (or trees) connecting routers having local mcast group members • tree: not all paths between routers used • source-based: different tree from each sender to rcvrs • shared-tree: same tree used by all group members

17. Approaches for building mcast trees Approaches: • source-based tree: one tree per source • shortest path trees • reverse path forwarding • group-shared tree: group uses one tree • minimal spanning (Steiner) • center-based trees …we first look at basic approaches, then specific protocols adopting these approaches

18. 1 i 5 4 3 6 2 Shortest Path Tree • mcast forwarding tree: tree of shortest path routes from source to all receivers • Dijkstra’s algorithm S: source LEGEND R1 R4 router with attached group member R2 router with no attached group member R5 link used for forwarding, i indicates order link added by algorithm R3 R7 R6

19. Reverse Path Forwarding • rely on router’s knowledge of unicast shortest path from it to sender • each router has simple forwarding behavior: if (mcast datagram received on incoming link on shortest path back to source) then flood datagram onto all outgoing links else ignore datagram

20. Building the Reverse Path source

21. Building a Reverse Path Tree source

22. Reverse Path Forwarding: example S: source LEGEND R1 R4 router with attached group member R2 router with no attached group member R5 datagram will be forwarded R3 R7 R6 datagram will not be forwarded • result is a source-specific reverse SPT • may be a bad choice with asymmetric links

23. Reverse Path Forwarding: pruning • forwarding tree contains subtrees with no mcast group members • no need to forward datagrams down subtree • “prune” msgs sent upstream by router with no downstream group members LEGEND S: source R1 router with attached group member R4 router with no attached group member R2 P P R5 prune message links with multicast forwarding P R3 R7 R6

24. Shared-Tree: Steiner Tree • Steiner Tree: minimum cost tree connecting all routers with attached group members • problem is NP-complete • excellent heuristics exists • not used in practice: • computational complexity • information about entire network needed • monolithic: rerun whenever a router needs to join/leave

25. Center-based trees • single delivery tree shared by all • one router identified as “center” of tree • to join: • edge router sends unicast join-msg addressed to center router • join-msg “processed” by intermediate routers and forwarded towards center • join-msg either hits existing tree branch for this center, or arrives at center • path taken by join-msg becomes new branch of tree for this router

26. Center-based trees: an example Suppose R6 chosen as center: LEGEND R1 router with attached group member R4 3 router with no attached group member R2 2 1 R5 path order in which join messages generated R3 1 R7 R6

27. Current Intra-Domain Multicast Routing Protocols DVMRP — Distance-Vector Multicast Routing Protocol flood-and-prune,unidirectional per-source trees,builds own routing table MOSPF — Multicast Extensions to Open Shortest-Path First Protocol broadcast membership,unidirectional per-source trees,uses OSPF routing table

28. Current Intra-Domain Multicast Routing Protocols (cont.) PIM-DM — Protocol-Independent Multicast, Dense-Mode broadcast-and-prune,unidirectional per-source trees,uses unicast routing table (Protocol Independent) PIM-SM — Protocol-Independent Multicast, Sparse-Mode uses meeting places (“rendezvous points”),unidirectional per-group or shared trees,uses unicast routing table (Protocol Independent) CBT — Core-Based Trees uses meeting places (“cores”),bidirectional shared trees,uses unicast routing table

29. The First Intra-Domain Routing Protocol: DVMRP

30. Subnet (Destination) shortest dist (cost) via interface (NextHop) a 1 i1 b 5 i1 c 3 i2 … … … Distance-Vector Multicast Routing Protocol (DVMRP) DVMRP consists of two major components: • a conventional distance-vector routing protocol (like RIP) which builds, in each router, a routing table like this: (2) a protocol for determining how to forward multicast packets, based on the routing table and routing messages

31. Example Topology g g s g

32. Phase 1: Truncated Broadcast g g s flood iff the packet arrives over the link that is on the shortest path to S g

33. Phase 2: Pruning g g prune (s,g) prune (s,g) s g

34. Steady State g g g s g

35. Grafting on New Receivers g g g report (g) graft (s,g) graft (s,g) s g

36. Steady State after Grafting g g g s g

37. Multicast Routing: MOSPF

38. Multicast OSPF (MOSPF) • an extension to OSPF (Open Shortest-Path First),a link-state, intra-domain routing protocol specified in RFCs 1584 & 1585 • multicast-capable routers indicate that capability with a flag in their link-state messages • routers include in their link-state messages a list of all groups that have members on the router’s directly-attached links (as learned through IGMP)

39. Link state: each router floods link-state advertisement Multicast: add membership information to “link state” Each router then has a complete map of the topology, includingwhich links have members of which multicast groups S1 Z X R1 Y R2

40. Z has network map, including membership at X and Y Z computes shortest path tree from S1 to X and Y Z builds multicast entry with one outgoing interface W, Q, R, each build multicast entries S1 Z W Q X R R1 Y R2

41. Link-state advertisement with new topology (may be due to link failure) may require recomputation of tree and forwarding entry. Link WZ failed in the diagram below. S1 Z W Q X R R1 Y R2

42. Link state advertisement (T) with new membership (R3) may require incremental computation and addition of interface to outgoing interface list (Z) (Similarly, disappearance of a membership may cause deletion an interface from an outgoing interface list). Link WZ is back to normal. S1 Z R3 W T Q X R R1 Y R2

43. Multicast Routing: PIM

44. Protocol Independent Multicast (PIM) • “Protocol Independent” • does not perform its own routing information exchange • uses unicast routing table made by any of the existing unicast routing protocols • PIM-DM (Dense Mode) - similar to DVMRP, but: • without the routing information exchange part • differs in some minor details • PIM-SM (Sparse Mode), or just PIM - instead of directly building per-source, shortest-path trees: • initially builds a single (unidirectional) tree per group , shared by all senders to that group • once data is flowing, the shared tree can be converted to a per-source, shortest-path tree if needed

45. PIM Protocol Overview • Basic protocol steps • routers with local members send Join messages towards a Rendezvous Point (RP) to join shared tree • routers with local sourcesencapsulate data to RP • routers with local members may initiate data-driven switch to source-specific, shortest-path tree

46. Phase 1: Build Shared Tree Shared tree after R1,R2,R3 join Join messagetoward RP Join G RP R1 R4 R2 R3

47. Phase 2: Sources Send to RP S1 unicast encapsulated data packet to RP RP decapsulates, forwards downShared tree PIM Register S2 RP R1 R4 R2 R3

48. Phase 3: Stop Encapsulation S1 (S1,G) (S1,G) S2 (S2,G) Join G for S1 Join G for S2 RP (*.G) R1 R4 R2 R3

49. Phase 4: Switch to Shortest Path Tree S1 shared tree S2 Join messagestoward S2 RP R1 R4 R2 R3

50. Phase 5: Prune (S2 off) Shared Tree S1 Prune S2 off Shared tree where iif of S2 andRP entries differ S2 distribution tree Shared tree S2 RP R1 R4 R2 R3